This invention relates to a stepless speed change device for a general purpose of being suitable for use in industrial machines, transportation facilities and the like.
There are various systems of stepless speed change devices. The friction stepless speed change devices concerning with the invention are classified into cone, disc, ring and spherical surface transmissions.
With any of the friction stepless speed change devices above described, the stepless speed change transmission is effected by changing rotating radii of the friction transmission contacting points in a stepless manner. The friction transmission contacting points are divided into two kinds of external and internal contacting types.
The external contacting types are disclosed for example, in Japanese Utility Model Application Publication No. 49-29,168 and Japanese Patent Application Publication No. 46-42,249. The internal contacting type is disclosed, for example, in Japanese Utility Model Application Publication No. 46-34,919.
In the external contacting type, the transmission is accomplished by contact between two convex surfaces, whose contacting surfaces are wide, belt-like surfaces along contacting orbits corresponding to pitch lines due to contacting pressures. As a result, positive and negative slips occur on outer and inner sides of the contacting orbit so that such slips result in internal friction losses which lower the transmission efficiency.
Moreover, the case of which either the driving or driven sides has a small curvature rotor, when the difference in rotating radii of the driving and driven sides becomes large, the above positive and negative slips further rapidly increase. As a result, the transmission efficiency is further lowered.
Therefore, the stepless speed exchange device of the external contacting type has a disadvantage in that the transmission efficiency is low due to the external contact.
On the other hand, with the device of the internal contacting type, the transmission is effected by contact between a concave surface and a convex surface. Contacting orbits are narrow, line-shaped surfaces. Therefore, the device of the internal contacting type is superior, having less internal friction losses and high transmission efficiency, to the device of the external contacting type.
With the device of the internal contacting type, however, the friction discs are apt to open out of a parallel position such it is difficult to maintain the driving and driven discs in parallel with each other and the contacting pressure becomes unstable, which results in a difficult transmission of torque and a lowered transmission efficiency.
Moreover, with a device of the internal contacting type, either the input or the output shaft is movable, so that in order to support the movable shaft a rigid member is needed.